1. Field of the Invention
The present invention relates to a cooking appliance of the so-called forced hot air circulation type wherein hot air heated by a heat source is fed into a heating chamber and the temperature distribution in the heating chamber is kept uniform by a circulation fan.
2. Description of the Prior Art
This type of cooking appliance includes an electric oven using an electric heater as a heat source, a gas oven using gas combustion as a heat source, and a composite cooking appliance comprising a microwave oven combined with such an oven.
While the demand for energy conservation has been gaining momentum these years, the gas oven suffers a great heat loss involved in exhaust peculiar to gas combustion, being inferior in thermal efficiency to the electric oven. Further, since this high temperature exhaust is discharged outside the appliance, severe restrictions are imposed on the gas oven relative to its surroundings from the standpoint of fire prevention.
In such circumstances, examples of gas ovens on the market will now be described with reference to FIGS. 1(a)-(b) and 2(a)-(c).
In a gas oven shown in FIG. 1, the front of a heating chamber 4 for heating a heating load 3 placed in a pan 2 is provided with a door 1. Disposed under the heating chamber 4, there are a burner 5 for gas combustion as a heat source and a combustion chamber 6 having a sufficient space for gas combustion. Disposed in the rear of the heating chamber 4, there is an air blast chamber 8 having a circulation fan 7 installed therein to feed hot air heated in the combustion chamber 6 and to keep uniform the temperature distribution in the heating chamber 4. In the rear of the air blast chamber 8, there is a combustion passage chamber 6' communicating with the combustion chamber 6 to introduce the hot air into the air blast chamber 8.
The circulation fan 7 is in the form of a disk having vanes 10 and 13 mounted thereon and is driven for rotation by a motor 9. The vanes 10 serve to draw the hot air, which has been introduced into the combustion passage chamber 6' from the combustion chamber 6, into the air blast chamber 8 through a suction port 11 and then deliver it into the heating chamber 4 through blast holes 12. Meanwhile the vanes 13 serve to draw the hot air into the air blast chamber 8 from the heating chamber 4 through vent holes 14 and then deliver it again into the heating chamber 4 through the blast holes 12.
The air supply and exhaust system necessary for gas combustion comprise air supply holes 16 for supplying air necessary for gas combustion effected by the burner 5 in the combustion chamber 6, and exhaust holes 15 formed in the upper region of the rear wall of the heating chamber 4; thus, the hot air forced out through the exhaust holes 15 passes through an exhaust passage 17 and then through a ceiling exhaust holes 18 for discharge from the outside.
Further, this gas oven is designed so that when the temperature in the heating chamber 4 reaches a preset value, the supply of gas to the burner is rendered intermittent to keep the temperature in the heating chamber 4 substantially constant, but the circulation fan 7 is allowed to continue rotating to ensure that the temperature distribution in the heating chamber 4 is uniform.
The conventional gas oven of FIG. 1 constructed in the manner described above feeds a substantially constant amount of air at all times into the heating chamber 4 from the combustion chamber 6 through the air blast chamber 8 by means of the rotation of the fan 7, so that it follows that the air pressure in the heating chamber 4 increases and that a substantially constant amount of exhaust is discharged through the upper exhaust holes 15.
Therefore, even when the burner 5 is in the combustion-off state during its intermittent or on-off operation started after the temperature in the heating chamber 4 has reached a predetermined value, the same amount of exhaust as that with the burner 5 in the combustion-on state is discharged; thus, discharge of exhaust, which is not necessary when the burner 5 is in the combustion-off state, is forced to take place, resulting in a great heat loss. This heat loss occurs because the circulation fan 7 is separately provided with the vanes 10 for drawing hot air from the combustion chamber 6 and the vanes 13 for circulating the hot air in the combustion chamber 4 and that the exhaust holes 15 are provided in the heating chamber 4 whose air pressure is always higher than the atmospheric pressure.
The gas oven shown in FIG. 2 has a construction in which the circulation fan 7 of the gas oven shown in FIG. 1 is improved. The combustion chamber 6 is located between the heating chamber 4 and the air blast chamber 8. The function of drawing the hot air heated in the combustion chamber 6 into the air blast chamber 8 through the suction port 11 and delivering it to the heating chamber 4 through the blast holes 12, and the function of drawing the hot air into the air blast chamber 8 from the heating chamber 4 successively through the vent holes 14, combustion chamber 6 and suction port 11 and delivering it to the heating chamber 4 through the blast holes 12 are performed by the vanes 13 alone.
Other arrangements and functions are the same as those of the gas oven shown in FIG. 1.
As a result of changing the position of the combustion chamber 6 and the arrangement of the circulation fan 7 in this manner, when the burner 5 is in the combustion-off state during its on-off operation after the temperature in the heating chamber 4 has reached a preset value, the combustion chamber 6 is substantially filled with the hot air fed thereto from the heating chamber 4 through the vent holes 14 and said hot air is drawn into the air blast chamber 8 through the suction port 11, so that the amount of cool air newly drawn through air feed holes 16 is relatively small and hence the amount of exhaust discharged through the exhaust holes 15 in the upper region of the heating chamber 4 correspondingly decreases. In the combustion-on period, since the combustion chamber 6 is substantially filled with combustion gas produced by combustion at the burner 5, the amount fed into the combustion chamber 6 from the heating chamber 4 decreases and hence the amount discharged through the exhaust holes 15 correspondingly increases. However, since the air pressure in the heating chamber 4 is substantially high even during the combustion-off period, the drawbacks that a substantial amount of exhaust is forced to take place and that the heat loss involved in exhaust is great, remain to be eliminated also in this conventional example.